This study examines the role of monocyte prostaglandins in regulating the proliferation, differentiation and function of T-lymphocyte precursors and mature cells during bone marrow engraftment. Patients undergoing bone marrow transplantation as part of their treatment for leukemia will be studied periodically during marrow engraftment. The T-lymphocyte precursor pool will be assessed by following CFU-T1 proliferation in soft agar with the T-colony formation method. Single colonies of T\cells will be isolated and grown to sufficient cell numbers to allow surface marker and functional studies to be carried out. Changes in T-cell subpopulations during engraftment in patients receiving cyclosporin\A treatment will be compared to patients who receive conventional immunosuppressive therapy. Changes in T-cell subpopulation frequencies will be sought in those patients in whom GVH reactions develop. We hope in such a study to elucidate the possible role of the monocyte/macrophage in T-cell ontogeny during marrow engraftment and the effect that cyclosporin\A has on this ontogeny.